The present invention relates to the deposition of a thin film and specifically to semiconductor thin film processing.
Deposition is one of the basic fabrication processes of modern semiconductor device structures. Deposition techniques includes Physical Vapor Deposition (PVD, or sputtering), and Chemical Vapor Deposition (CVD) and numerous variations of CVD such as pulsed CVD, sequential CVD or Atomic Layer Deposition (ALD). PVD process uses a high vacuum apparatus and generated plasma that sputters atoms or clusters of atoms toward the surface of the wafer substrates. PVD is a line of sight deposition process that is more difficult to achieve conform film deposition over complex topography such as deposition of a thin and uniform liner or barrier layer over the small trench or via of 0.13 μm or less, especially with high aspect ratio greater than 4:1.
CVD method is different from PVD method. In CVD, a gas or vapor mixture is flowed over the wafer surface at an elevated temperature. Reactions then take place at the hot surface where deposition takes place. The basic characteristic of CVD process is the reaction at the substrate of all the precursors vapors together. The reaction often requires the presence of an energy source such as thermal energy (in the form of resistive heated substrate, or radiative heating), or plasma energy (in the form of plasma excitation). Temperature of the wafer surface is an important factor in CVD deposition, because the deposition depends on the reaction of the precursors and affects the uniformity of deposition over the large wafer surface. CVD typically requires high temperature for deposition which may not be compatible with other processes in the semiconductor process. CVD at lower temperature tends to produce low quality films in term of uniformity and impurities. The reactions can be further promoted by plasma energy in plasma enhanced CVD process, or by photon energy in rapid thermal CVD process. CVD technology has been used in semiconductor processing for a long time, and its characteristics are well known with a variety of precursors available. However, CVD process needs major improvements to meet modern technology requirements of new materials and more stringent film qualities and properties.
Variations of CVD include pulse CVD or sequential CVD. In pulse or sequential CVD, the chemical vapors or the supplied energies such as plasma energy, thermal energy, laser energy are pulsed instead of continuous as in CVD process. The major advantages of pulse CVD is the high effects of the transient state resulted from the on-off status of the precursors or the energies, and the reduced amount of precursors or energies due to the pulsed mode. Reduced energy is desirable which can be accomplished in pulsed mode since it leads to less substrate damage such as the case of plasma processing for thin gate oxide. Reduced precursor is desirable which can be accomplished in pulsed mode for specific applications such as epitaxial deposition where the precursors need to react with the substrate in an arrangement to extend the single crystal nature of the substrate. There are no purging steps in pulsed CVD since cross contaminations or gas phase reactions are not a concern, and the purpose of the pulsing of the precursors or energies is to obtain the desired film characteristics.
Pulsed CVD can be used for create gradient deposition such as U.S. Pat. No. 5,102,694 of Taylor et al. Taylor discloses a pulsed deposition process in which the precursors are periodically reduced to create a gradient of composition in the deposited films. Taylor's pulsed CVD relies only on the changing of the first set of precursors to vary the film compositions.
Pulsed CVD can be used to modulate the precursors flow such as U.S. Pat. No. 5,242,530, titled “Pulsed gas plasma-enhanced chemical vapor deposition of silicon”, of Batey et al. Batey discloses a pulsed deposition process in which the precursor silane is modulated during a steady flow of plasma hydrogen. The pulsing of silane creates a sequence of deposition and without the silane pulses, the steady plasma hydrogen cleans and prepare the deposited surface.
Pulsed CVD can be used to pulse the plasma energy needed for the deposition process such as U.S. Pat. No. 5,344,792, titled “Pulsed plasma enhanced CVD of metal silicide conductive films such as TiSi2, of Sandhu et al. Sandhu discloses a pulsed deposition process in which the precursors are introduced into a process chamber, then the plasma energy is introduced in pulsed mode to optimize the deposition conditions. U.S. Pat. No. 5,985,375, titled “Method for pulsed plasma enhanced vapor deposition”, of Donohoe et al. discloses a similar pulsed CVD process with the plasma energy in pulsed mode but with a power-modulated energy waveform. The pulsing of the plasma energy allows the deposition of a metal film with desired characteristics. U.S. Pat. No. 6,200,651, titled “Method of chemical vapor deposition in a vacuum plasma processor responsive to a pulsed microwave source”, of Roche et al. discloses a pulsed CVD process with an electron cyclotron resonance plasma having repetitive pulsed microwave field to optimize the deposited films. U.S. Pat. No. 6,451,390, titled “Deposition of TEOS oxide using pulsed RF plasma”, of Goto et al. discloses a TEOS oxide deposition process using a pulsed RF plasma to control the deposition rate of silicon dioxide. The pulsing feature offers the optimization of the deposit films through the transient state instead of the steady state. Pulsing of plasma during nitridation process of gate oxide shows less damage than continuous plasma nitridation process because of higher interaction due to plasma transient state and less damage due to shorter plasma time.
Pulsed CVD can be used to pulse the precursors needed for the deposition process such as U.S. Pat. No. 6,306,211, titled “Method for growing semiconductor film and method for fabricating semiconductor devices”, of Takahashi et al. Takahashi discloses a pulsed CVD process to deposit epitaxial film of SixGeyCz. Epitaxial deposition requires a single crystal substrate, and the deposited film extends the single crystal nature of the substrate, different from CVD poly crystal or amorphous film deposition. To extend the single crystal nature of the substrate, the deposited precursors need to bond with the substrate at specific lattice sites, therefore a reduced precursor flow is highly desirable in epitaxial deposition to allow the precursors enough time to rearrange into the correct lattice sites. The process includes a continuous flow of hydrogen to dilute the precursors to be introduced. Then sequential pulses of silicon-based precursor, germanium-based precursor and carbon-based precursor are introduced to deposit an epitaxial film of SixGeyCz. To deposit epitaxial film, little amounts of precursors are needed, and this can be accomplished by short pulses (order of micro seconds) and further diluted in high flow of hydrogen. Takahashi discloses that the pulses of the precursors are not overlaped, but is silent on the separation of these pulses. The objective of Takahashi pulsed CVD is to deposit compound films, therefore the separation of these precursors is not relevant.
Pulsed CVD as described by Takahashi et al. to deposit epitaxial film of SixGeyCz, does not allow deposition of high coverage or conformal film on a non-flat substrate, such as in a via or trench for interconnects in semiconductor devices. The objective of Takahashi pulsed CVD is to deposit epitaxial films with sufficiently planar surface as observed by Takahashi et al., without mentioning of possible deposition on trenches or vias.
ALD is another variation of CVD using chemical vapor for deposition. In ALD, various vapors are injected into the chamber in alternating and separated sequences. For example, a first precursor vapor is delivered into the chamber to be adsorbed on the substrate, then the first vapor is turned off and evacuated from the chamber. Another precursor vapor is then delivered into the chamber to react with the adsorbed molecules on the substrate to form a desired film. Then this vapor is turned off and evacuated from the chamber. This sequence is repeated for many cycles until the deposited film reaches the desired thickness. There are numerous variations of ALD processes, but the ALD processes all share two common characteristics: sequentially precursor vapors flow and self-limiting thickness per cycle. The sequential precursor flow and evacuation characteristic offers the elimination of gas phase reaction commonly associated with CVD process. The self-limiting thickness per cycle characteristic offers the excellent surface coverage, because the total film thickness does not depend on precursor flow, nor on process time. The total film thickness depends only on the number of cycles. The ALD process then is not sensitive to the substrate temperature.
The maximum thickness per cycle of ALD process is one monolayer because of the self limiting feature that the substrate surface is saturated with the first precursor. The first precursor can adsorb on the substrate, or the first precursor can have some reaction at the substrate, but the first precursor also saturate the substrate surface and the surface is terminated with a first precursor ligand.
The throughput of ALD process depends on how fast a cycle is, and therefore a small chamber volume is critical. Furthermore, the fast switching of the precursor valves is desirable to allow a high throughput. A typical ALD cycle is a few seconds long, therefore the precursor pulses are in order of second. Precursor depletion effect can be severe for this short process time.
U.S. Pat. No. 5,916,365 to Sherman entitled “Sequential chemical vapor deposition” provides for sequential chemical vapor deposition (ALD) by a sequence of chamber evacuating, adsorption of the first precursor onto the substrate, then another chamber evacuation, then a second radical precursor to react with the adsorbed precursor on the substrate surface, and a third chamber evacuation. The Sherman process produces sub-monolayers per cycle due to adsorption. The process cycle can be repeated to grow the desired thickness of film. Sherman discloses an ALD process in which the first precursor process flow is self-limiting, meaning no matter how long the process is, the adsorption thickness cannot changed. U.S. Pat. No. 6,015,590 to Suntola et al., entitled “Method for growing thin films” discloses an ALD process which completely separates the precursors. Suntola process is an improved ALD process (called ALE by Suntola) meaning the deposition is achieved through the saturation of precursors on the substrate surface and the subsequent reaction with the reactants. The advantage of Suntola process is the complete separation of precursors, with a better than 99% purging between pulses of precursors to prevent cross reactions.
U.S. Pat. No. 6,200,893, and its divisions (U.S. Pat. Nos. 6,451,695, 6,475,910, U.S. patent publication 2001/0002280, U.S. patent publication 2002/0192954, U.S. patent publication 2002/0197864) to Sneh entitled “Radical-assisted sequential CVD” discusses a method for ALD deposition. Sneh sequence process is a variation of ALD process. Sneh discloses a deposition step for the first precursor introduction, but the deposition of Sneh is self limiting because of the surface saturation with ligands. In fact, in U.S. Pat. No. 6,475,910, Sneh discloses a method to extend the thickness of the first precursor introduction step. The disclosure discloses another ALD process to sequential precursor flows to increase the thickness of the first precursor introduction step. In a way, this is similar to a nested loop, where the thickness of the first precursor flow step of an ALD process can be increased by another ALD process.